Dissertation Defenses

Doctoral students who have an upcoming dissertation oral defense are posted here. So why not take this opportunity to learn about the research that our graduate students are doing!

Dissertation Defense for Yunyao Jiang

Program: MECHANICAL ENGR: PHD

Department Contact Email: jamie.mcwilliams@unh.edu

Defense Title: DESIGN, MECHANICAL EXPERIMENTS AND MODELING ON A NEW FAMILY OF 3D PRINTED HYBRID CHIRAL MECHANICAL METAMATERIAL WITH NEGATIVE POISSON’S RATIO

Defense Date and Time: 11/20/18 10:00 am

Defense Location: Kingsbury W290

Defense Advisor: Prof Yaning Li

Defense Abstract: Emerging in the last decade, mechanical metamaterials have many engineering advantages due to unusual mechanical properties. Auxetic (with negative Poisson’s ratio) mechanical metamaterial, as an important family of these materials, have broad engineering applications including the design of new materials with increased indentation resistance, shear resistance, energy absorption capability and variable permeability.
Chiral auxetic mechanical metamaterial is a sub-family of the auxetic material, which is with a geometry that is non-superposable on its mirror image. Compared to symmetric auxetic materials, such as re-entrant honeycombs or periodic porous materials, which have random handedness after instability, chiral materials have deterministic handedness and therefore are expected to have more robust auxetic effects under manufacturing errors with both small and large deformations.
Moreover, due to the chirality, the deformation mechanisms of this sub-family of auxetic material are quite interesting: for 2D cases, there will be shear-compression/tension coupling effects; and for 3D cases, there will be twist-compression/tension coupling effects.
In this dissertation, through innovative 2D and 3D designs, a new family of auxetic chiral mechanical metamaterials were systematically developed. Besides auxetic effects, several new deformation mechanisms were designed and thoroughly investigated, such as a sequential cell opening mechanisms, a rotation-induced chiral pattern transformation triggered either by mechanical instability or shape memory effects, and shear/twist-compression/tension coupling effects. To proof the concept, multi-material 3D printing was extensively used to fabricate hybrid designs for both uniaxial and bi-axial compression/tension experiments. A novel bi-axial compression apparatus was designed and fabricated via 3D printing to perform bi-axial compression on a uniaxial material testing machine.
To better understand the mechanical behaviours of these materials and predict the mechanical properties of the designs, both sophisticated structural model and advanced continuum models such as monoclinic material model and micropolar elasticity are used for mechanical modelling.
In summary, through the innovative designs, the database of existing chiral auxetic metamaterials is significantly expanded which expedites the discovery of new mechanical properties and behaviours. Also, the extended database enables the development of advanced constitutive models either within classic continuum theory or beyond.

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